Manufacturer · Fashion & Home Accessories
Sewing machine thread tension is the balance between the force pulling the needle thread and the bobbin thread from the hook. When the balance is right, the two threads interlock exactly in the middle of the fabric and the seam looks the same on both sides. If there are loops underneath, the upper tension is too weak; if the bobbin thread shows on top, the upper tension is too strong.
How to read the seam
Before turning the dial, look carefully at the seam on both sides. It shows exactly what is wrong.
| What you see | Probable cause | What to do |
|---|---|---|
| Loops underneath the seam | Weak upper tension, or the thread is not between the discs | Rethread with the foot raised, then tighten the upper tension |
| The bobbin thread shows as dots on top | Strong upper or weak bobbin tension | Reduce the upper tension; check the bobbin |
| The needle thread lies as a straight line underneath | Very strong bobbin tension | Loosen the bobbin case screw |
| The fabric puckers along the seam | Overall tension too strong or thread too thick | Reduce the tension, check the thread and stitch |
| The seam breaks when stretched | Strong tension and too little thread in the stitch | Loosen the tension, lengthen the stitch or choose a stretch stitch |
Why loops appear underneath the seam
This is the most common complaint, and in most cases it has nothing to do with the bobbin thread. When the needle thread has not gone between the tension discs, the machine does not tension it at all and it forms loops on the wrong side. The cause is almost always threading with the foot lowered: when lowered, the discs close and the thread passes beside them. Raise the foot, remove the thread and rethread the entire upper path.
Other causes of loops underneath:
- the thread has come out of the take-up lever (the lever that moves up and down);
- dust or fibres between the tension discs;
- the thread is caught in the notch of the spool and pulls in jerks;
- a thread that is too fine or slippery at low tension.
How to set sewing machine thread tension
- Start with the needle thread. On most domestic machines, the middle setting of the dial is the starting point. Industrial machines have a tensioner nut that is turned in small steps.
- Use contrasting colours for the test. Put a different colour on top and bottom on a piece of the same fabric folded in two layers.
- Change only one thing at a time. Turn the dial by one step, sew 10 cm and compare.
- Adjust the bobbin tension last. The bobbin case screw is turned in minimal steps – a quarter turn is a big change. Note the starting position.
- Check at actual speed. In the factory, the seam may look different in a slow test and at full speed.
The hanging bobbin case test
On industrial machines, bobbin tension is often checked like this: hold the end of the thread and let the bobbin case hang. When jerked lightly, it should drop slowly by a few centimetres and stop. If it falls freely, the tension is weak; if it does not move, it is too strong.
When the thread is to blame
If the machine is set correctly but the seam is still uneven, check the thread. The quality of sewing threads directly affects tension:
- Uneven thickness. Thick spots pass through the discs with more friction and the seam becomes wavy.
- Different threads top and bottom. A cotton needle thread and a polyester bobbin thread differ in slip and elasticity. Balance is hard to achieve.
- Unsuitable thickness for the needle. A thick thread in a thin needle rubs and the tension rises unpredictably.
- No finish (lubricant). Industrial threads have a smooth coating that reduces friction. Without it, the thread "squeaks" in the tensioner.
- Old thread. Dried-out thread loses elasticity and the seam puckers.
Bobbin winding and tension
Many bobbin tension problems start with winding. The bobbin must be wound evenly, without loose layers and not right up to the rim. Do not wind new thread over the remains of another. Polyester thread wound too tightly and quickly stretches and then contracts on the bobbin, which changes the tension. When changing the bobbin, also check the unwinding direction given in the machine manual.
Puckering on thin fabrics
On fine fabrics such as chiffon, satin and shirting, puckering is rarely fixed by tension alone. A finer thread (ticket 120–180), a thin needle, a shorter stitch and reduced presser foot pressure help. In series sewing, the difference in thread shows most clearly: a fine core-spun or spun polyester gives a flatter seam than a thick thread used "for strength". For natural fabrics, take a look at our cotton threads too.
Tension on overlock and chainstitch machines
On an overlocker, each thread has its own dial, and the balance is judged along the edge of the fabric. If the looper loops hang over the edge, tighten the upper looper; if the needle threads form a "ladder" when the seam is pulled open, tighten the needles. The rule is the same here: one change at a time, a test on the same fabric and a record of the settings that give a clean seam, so that you can use them for the next batch.
Threads from Polkom for an even seam
Polkom is a Bulgarian manufacturer and supplier of garment accessories. We offer sewing threads wholesale for industrial and domestic machines, as well as the full range of threads for sewing and embroidery. If your factory has recurring tension problems, send us the type of fabric, the machine and the thread you use – we will suggest a suitable thread and a sample for testing. Inquiries can be sent via our contact page.
